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Sa Memvber, Onzole Formation (Pliocene of Ecuador)

Where: Esmeraldas, Ecuador (0.9° N, 79.9° W: paleocoordinates 0.7° N, 79.1° W)

• coordinate estimated from map

• local area-level geographic resolution

When: Onzole Formation (Daule Group), Early/Lower Pliocene (5.3 - 3.6 Ma)

• LITHOSTRATIGRAPHY: From the Sa Member of the Onzole Fm of the Daule Group, which overlies the lower member of the Onzole Fm (Late Miocene) and the Angostura Fm (Late Miocewne). AGE: Early Pliocene in text. STRATIGRAPHIC POSITION: Composite list 5070-cm-thick shellbed at the base of the Su ́a Member.

• bed-level stratigraphic resolution

Environment/lithology: coastal; burrowed, bioturbated, silty sandstone

• lthough composed of largely autochthonous-parautochthonous specimens, the fossil assemblage contains mollusk species from a range of water depths and both soft- and hard-bottom habitats, implying in situ time averaging and admixture of different assemblages as transgression proceeded on a sediment-starved shoreline. Known depth ranges of the majority of the extant species, among which A. reinharti and O. splendidula are the dominant elements, point to deposition within an upper shoreface environment, even though a very few taxa, such as Chama pellucida and Ostrea iridescens, inhabit the foreshore zone; however, their presence in the assemblage may be interpreted as related to the substrate requirements rather than to water depth tolerances. High abundance at the outcrop level of the penshell A. maura is consistent with a shoreface setting, probably no more than 2030 m in water depth. The most abundant species within this group (T. altilira altilira) is the prominent element of an upper shoreface fossil assemblage from the Miocene Angostura Formation (Cantalamessa et al., 2007). Shellbeds of this type, immediately overlying a Glossifungites-demarcated ravinement surface, are regarded as onlap shellbeds and record landward stratal convergence and attenuation during the earliest phases of marine transgression
• LITHOLOGY: Bulk samples consist of poorly lithified sediments (i.e., affected by incipient cementation only). LITHIFICATION: 5070-cm-thick, mollusk-dominated fossiliferous horizon including widely scattered rip-up clasts with abundant macroborings by lithophagid bivalves (Gastrochaenolites) (Fig. 2B). In section, this shellbed has a simple internal architecture in which the bioclastic components, which are packed densely to loosely in the lower part, pass rapidly upward into a dispersed, matrix-supported biofabric (Fig. 2C). The embedding matrix consists of massive, silty sandstones thoroughly bioturbated by Thalassinoides burrows that form an irregularly shaped network of tunnels and wider chambers.

Size class: macrofossils

• The bioclasts from the bulk sample show a high degree of fragmentation and include a huge number of shell fragments.

Preservation: original aragonite

Collection methods: bulk, surface (in situ), sieve,

• COLLECTOR: By the authors, although not specified. REPOSITORY: Not stated in text. METHODS: Bulk samples, amalgamated from multiple replicates (,30 dm3 each) were collected from different lateral spots of the same fossiliferous horizon, and manual picking at outcrop level, in order to obtain large and rare species. Bulk samples consist of poorly lithified sediments (i.e., affected by incipient cementation only, with rocks easily disintegrable with hydrogen peroxide) that were soaked in a strongly diluted H2O2 and water solution and later washed with fresh water, air dried, and gently sieved through 1 mm mesh screens. Slightly < 1900 fish otoliths were extracted from a 1000 kg cumulative bulk sample amalgamated from multiple replicates collected adjacently over the entire exposed length of the same horizon (lateral spacing between the sampling sites is 100 m or less). Bulk material was reduced to a reasonable size after processing with water, air drying, and sieving (smallest screen 0.5 mm) and otoliths were separated from the final residue by hand picking.

Primary reference: G. Carnevale, W. Landini, L. Radgaini, C. Celma, G. and Cantalamessa. 2011. Taphonomic and paleoecological analyses (mollusks and fishes) of the Sa Member condensed shelled, upper Onzole Formation (Early Pliocene, Ecuador). Palaios 26(3):160-172 [A. Hendy/A. Hendy/P. Holroyd] more details

Purpose of describing collection: paleoecologic analysis

PaleoDB collection 135043: authorized by Austin Hendy, entered by Austin Hendy on 20.10.2012

Creative Commons license: CC BY (attribution)